β-Stereoselective Mannosylation Using 2,6-Lactones

β-Stereoselective mannosylation using donors bearing the 2,6-lactone moiety is described. In general, glycosylation is a nucleophilic substitution reaction between an alcoholic nucleophile and a sugar moiety containing a leaving group at the anomeric position. Owing to stereoelectronic effects, the reaction tends to proceed via an SN1 mechanism to afford α-glycosides. We found that the introduction of a 2,6-lactone bridge can circumvent the competing SN1 reaction, affording β-glycosides with stereoinversion via SN2(-like) mechanisms. Glycosyl trichloroacetimidates are particularly efficient when activated by a combined catalyst of AuCl3 and 3,5-bis(trifluoromethyl)phenyl thiourea. In addition, the product stereoselectivity was highly dependent on the concentration of the reaction. Moreover, even when the reaction proceeds via an SN1 mechanism, the corresponding glycosyl cation appears to present sterically a β-directing nature. Overall, 2,6-lactones were promising structures for achieving β-mannosylations.